In surgery, either biostable or biodegradable devices are used for the fixation of bone fractures to immobilize the bone fragments and accelerate patient mobilization.
Most biostable devices are typically made of metallic alloys. See R. M. Pilliar, Powder Metal-Made Orthopaedic Implants With Porous Surface For Fixation By Tissue Ingrowth, Clinical Orthopaedics and Related Research, Vol. 176, 1983, pp. 42-51. Nevertheless, there are several disadvantages in the use of metallic implants. One such disadvantage is bone resorption caused by bone plates and screws, which carry most of the external loads, leading to stress protection produced by the modulus mismatch between metals and bone. Another disadvantage is the carcinogenic potential and the possibility of corrosion. Therefore, surgeons are recommended to remove metallic bone plates and screws in a second operation once the fracture has healed.
Bioresorbable polymeric fracture fixation devices have been studied as replacements for metallic implants. See S. Vainiopää, P. Rokkanen, P. Törmälä, Surgical Applications Of Biodegradable Polymers In Human Tissue, Progress in Polymer Science, Vol. 14, 1989, pp. 679-716. The advantages of these devices are that materials resorb in the body and degradation products disappear via metabolic routes. Hence, a second operation is not required. Additionally, the strength and the stiffness of the bioresorbable polymeric devices decreases when the device degrades and hence the bone is progressively loaded (which promotes bone regeneration). One disadvantage is the relatively low strength of existing polymeric devices. In the case of cortical, bone fracture, for example, unreinforced poly lactic acid (PLLA) plate and screws are too weak initially to permit patient mobilization. See J. Eitenmüller, K. L. Geriach, T. Scmickal, H. Krause, An In Vivo Evaluation Of A New High Molecular Weight Polylactide Osteosynthesis Device, European Congress on Biomaterials, Bologna Italy, Sep. 14-17, 1986, p. 94. In addition, the relatively low values of Young's modulus compared to metallic plates mean that thicker sections are required to ensure adequate stability.
Törmälä et al. have developed self-reinforced bioresorbable polymeric composites to improve the strength of bioresorbable polymer devices. These show good mechanical properties: e.g. bending strengths of 360±70 MPa and bending moduli of 12±2 GPa, respectively, have been reported. See P. Törmälä, Biodegradable Self-Reinforced Composite Materials; Manufacturing, Structure and Mechanical Properties, Clinical Materials, Vol. 10, 1992, pp. 29-34.
A common property of most polymeric implants is the lack of bony ongrowth to the material. In contrast, such bone apposition is produced by bioactive ceramics and glasses. See O. H. Andersson, K. H. Karlsson, Bioactive Glass, Biomaterials Today And Tomorrow, Proceedings of the Finnish Dental Society Days of Research, Tampere, Finland, 10-11 Nov. 1995, Gillot Oy, Turku, 1996, pp. 15-16. By adding bioactive ceramics or glasses to polymers to produce a composite, the bioactivity of the material can be improved. This effect has been demonstrated in dental composites and bone cement. See J. C. Behiri, M. Braden, S. N. Khorashani, D. Wiwattanadate, W. Bonfield, Advanced Bone Cement For Long Term Orthopaedic Applications, Bioceramics, Vol. 4, ed. W. Bonfield, G. W. Hastings and K. E. Tanner, Butterworth-Heinemann ltd, Oxford, 1991, pp. 301-307.
Bonfield et al have developed a biostable composite consisting of a polyethylene matrix and a particulate hydroxyapatite reinforcement (HAPEX™). See W. Bonfield, J. A. Bowman, M. D. Grynpas, UK Patent GB2085461, 1984. HAPEX™ composites show bioactivity above 0.20 volume fraction hydroxyapatite. See W. Bonfield, C. Doyle, K. E. Tanner, In Vivo Evaluation Of Hydroxyapatite Reinforced Polyethylene Composites, Biological and Biomechanical Performance of Biomaterials, ed., P. Christel, A. Meunier, A. J. C. Lee, Elsevier Science Publisher, 1986, pp. 153-158. Additionally, degradable composites of hydroxyapatite and copolymers of polyhydroxybutyrate and polyhydroxyvalerate have been described. See C. Doyle, K. E. Tanner, W. Bonfield, In Vitro And In Vivo Evaluation Of Polyhydroxybutyrate And Polyhydroxybutyrate Reinforced With Hydroxyapatite, Biomaterials, Vol. 12, 1991, pp. 841-847. The main limitation of these biostable and biodegradable composites is their inadequate mechanical strength for large bone fracture fixation. Also, use of hydroxyapatite and poly lactic acid composites has been reported. See Y. Ikada, H. H. Suong, Y. Shimizu, S. Watanabe, T. Nakamura, M. Suzuki, A. T. Shimamoto, Osteosynthetic Pin, U.S. Pat. No. 4,898,186, 1990. Using existing elements the composite still has quite moderate mechanical strength. Also in all these cases mentioned above the method of producing the composite differs from the method of the present invention.